The invention relates to a process for the wall ironing of a product in sheet form, which is formed from a metal sheet coated on at least one side with a layer of plastic, the wall-ironing tool comprising a forming surface which the product with a plastic coating layer moves along during the wall ironing, and the forming surface being at an entry angle with respect to the direction of movement of the product. A process of this nature is in widespread use for the production of a can comprising a base and a tubular body, although the invention is not limited to this particular application.
The entry angle forms an important parameter in wall ironing. It has been found that with a very small entry angle the spreading force, that is to say the force which acts on the forming surface transversely with respect to the direction of movement of the product, becomes very high. For example, in the case of wall ironing of cans, this may lead to extreme loads being imposed on the wall-ironing ring used, which may consequently be damaged or even break.
Selecting a larger entry angle runs the risk of the plastic layer breaking and being stripped off the metal sheet This is because a larger entry angle results in a greater longitudinal force being exerted on the plastic layer in the direction of movement, with the result that the stress in the said plastic layer exceeds a fracture limit.
Proposals have previously been made for making the process more suitable for working with plastic-coated metal sheet. In European Patent EP 0,298,560, it is proposed that additional lubrication be used during the wall ironing, and specific entry angles are proposed for successive wall-ironing rings. Nevertheless, there is a continuing need to work with larger entry angles, in order to be able to achieve longer service lives of the wall-ironing tool. The present invention now offers a solution enabling the risk of the plastic layer breaking and being stripped off during wall ironing to be reduced, so that larger entry angles can be used.